Stools incorporating backs have been familiar pieces of furniture for millenia, examples having been found in, for example, the tombs of the Pharaohs. The problems attendant to achieving a stable chair structure, including, stable legs and a stable back support have been known for many years and numerous solutions have been proposed.
Typical of the solutions which have been employed over the years, is the inclusion of the back and the rear legs as a single unitary structure, which, after the traditional securing of the legs to the seat results in a stable and durable back support. Unfortunately, this type of construction is most often used in a chair in which the back is formed by a plurality of members, which is relatively uncomfortable. The comfort of such backs, which include so-called ladder-back and comb-back chairs, may be improved by the addition of upholstery, at additional cost.
Yet another approach is to provide a back which is supported by diagonal support elements which extend from the back of the seat to an extension on the rear of the back support. Here however the application of forces during sitting is concentrated at the points of support and the longevity of the structure is thus impaired.
Generally, pressure on the back portion, such as caused by a person leaning back, acts at the points of attachment focusing the stress forces at this point. The stressing will consist of various twisting forces or moments caused by the forces (pressure on the back portion) acting through distance (the distance from the position where the pressure is placed on the back portion to the points of attachment).
The forces acting forward-to-back, up-and-down and twisting may fatigue the structural elements at the points of attachment or may loosen or weaken the joint at the point of attachment. Fatigue, weakening and loosening will affect the integrity of the chair resulting in "give", wobbling or shakiness and possibly failure.